DC9 CREW DEPARTED BIS BEFORE THE TWR OPENED.

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TWR AT BIS DOESN'T OPEN UNTIL XB00 LCL. CAPT AND I PICKED UP CLRNC OVER GFK FSS. WE PLANNED TO CONTACT ZMP BEFORE WE DEPARTED; BUT AT THE END OF THE RWY WE COULD NOT MAKE RADIO CONTACT. OUR FOM STATES THAT WE CAN DEPART VFR IF NO ATC SVC IS AVAILABLE. WE TOOK OFF WITHOUT INCIDENT AND CONTACTED ZMP. WE WERE RADAR IDENTED WHILE STILL VFR (APPROX 9000 FT MSL). THE QUESTION IS: SHOULD WE HAVE RECONTACTED GFK FSS TO OBTAIN DEP RELEASE? I PLAN TO CALL MY ACR TRAINING DEPT AND ASK. IN ANY CASE; I PLAN TO SEE WHAT CAN BE DONE TO HAVE INSTRUCTIONS MORE CLRLY WRITTEN; SECOND POSSIBLE PROB WAS THAT WE RECEIVED A RWY RPT. IT HAD JUST STOPPED SNOWING. WE WERE DEICED. WE COULD SEE A THIN (1/2 - 1 INCH) LAYER OF SNOW ON ALL ARPT SVCS AND ASSUMED THE RWY TO BE THE SAME. IT DID NOT DAWN ON US UNTIL AFTER TKOF THAT RWY COULD HAVE BEEN COVERED WITH MORE SNOW OR THAT A SNOW PLOW COULD HAVE BEEN ON RWY WITHOUT LIGHTS. WE WERE USING CTAF AND WITH A VISIBILITY OF OVER 3 MI COULD CLRLY SEE THE END OF THE RWY AND FELT SAFE (AT THE TIME) WITHOUT ALL OF OUR ACTIONS. I PLAN TO BRING THIS UP TO MY TRAINING DEPT. AT THIS POINT I FEEL WE SHOULD HAVE HAD THE RWY INSPECTED SOMEHOW. AT THE MOST IT WOULD HAVE BEEN A 45 MIN DELAY UNTIL THE TWR OPENED.